The invention relates to an optical waveguide fiber optimized for low attenuation. In particular, waveguide fiber attenuation is minimized for any core refractive index profile by proper selection of the core refractive index profile variables.
The dependence of waveguide properties upon the configuration of the refractive index profile has been described in the pioneering patent, U.S. Pat. No. 4,715,679, Bhagavatula. In that patent, core refractive index profiles are disclosed which provide for a variety of waveguide fiber properties, especially those having a zero dispersion wavelength shifted into the 1550 nm operating window and those which have a relatively constant dispersion over an extended wavelength range such as 1250 nm to 1600 nm.
In response to demands for specialized waveguide fibers, particularly with regard to high performance waveguides, investigation of waveguide core refractive index profiles has intensified. For example in U.S. Pat. No. 5,483,612, Gallagher et al., (the '612 patent) there is disclosed a core profile design which provides low polarization mode dispersion, low attenuation, a shifted dispersion zero, and low dispersion slope. Other core refractive index profiles have been designed to meet the requirements of applications which include the use of higher power signals or optical amplifiers.
A problem which may arise when a core profile is altered in order to arrive at a desired property is that the property is realized at the expense of another essential property. For example, a certain core refractive index profile design may provide increased effective area, thus reducing non-linear distortion of the signal. However, in this large effective area waveguide fiber, the bend resistance may be seriously compromised. Thus, core profile design is an exacting task, in which model studies usually precede the manufacturing stage of product development.
The interaction of the profile variables is such that one skilled in the art usually cannot, except perhaps in a very general way, predict the impact of a refractive index profile change upon such waveguide properties as, bend resistance, attenuation, zero dispersion wavelength, and total dispersion and total dispersion slope over a selected wavelength range. Therefore, studies of waveguide refractive index profiles usually include a computer simulation of the particular profile or family of profiles. Manufacturing testing is then carried out for those refractive index profiles which exhibited the desired properties.
In a continuation of the work disclosed in the '612 patent, a family of profiles was found which produced a high performance fiber having a zero dispersion wavelength above a pre-selected band of wavelengths and excellent bend resistance. A description of this work has been filed recently as a provisional application, Ser. No. 60/050550.
As further model studies and manufacturing tests were completed, it became clear that:
a particular family of profiles could be found to provide a selected set of operating parameters; and, most surprisingly, PA1 the profiles of the particular family could be further adjusted to optimize attenuation without materially changing the operating parameters. PA1 radius of the central core region is measured from the axial centerline of the waveguide to the intersection with the x axis of the extrapolated central index profile; PA1 radius of the second annular region is measured from the axial centerline of the waveguide to the center of the baseline of the second annulus; and, PA1 the width of the second annular region is the distance between parallel lines drawn from the half refractive index points of the index profile to the waveguide radius. PA1 .DELTA. %=[(n.sub.1.sup.2 -n.sub.c.sup.2)/2n.sub.1.sup.2 ].times.100, where n.sub.1 is a core index and n.sub.c is the minimum clad index. Unless otherwise stated, n.sub.1 is the maximum refractive index in the core region characterized by a % .DELTA.. PA1 n(r)=n.sub.0 (1-.DELTA.[r/a].sup..alpha.) where r is radius, .DELTA. is defined above, a is the last point in the profile, r is chosen to be zero at the first point of the profile, and I is a real number. For example, a triangular profile has .alpha.=1, a parabolic profile has .alpha.=2. When .alpha. is greater than about 6, the profile is essentially a step. Other index profiles include a step index, a trapezoidal index and a rounded step index, in which the rounding may be due to dopant diffusion in regions of rapid refractive index change.